The present invention relates to improvement of an electric power steering apparatus for assisting steering force by rotationally driving a motor for assisting steering force under PWM control with a target value of the motor current for the motor determined on the basis of the detected amount of the steering torque used as a target amount for automatic control.
In an electric power steering apparatus for rotationally driving a motor for assisting steering force under PWM control based on the current target value of the motor determined on the basis of the detected amount of steering torque and the detected amount of the driving current for the motor, in case of turning back a steering wheel, steering wheel return current is led to the motor to carry out the steering wheel return control. And, when the steering wheel returns to the position near the neutral position of the steering wheel (steering angle middle point) by which the vehicle is advanced straight, the steering wheel return current is set to zero.
However, even when the steering wheel returns to its neutral position, due to the inertia force of the motor, the steering wheel does not instantly stop rotating in the position but repeats going beyond the neutral position (0.degree.) and then turning back like a pendulum until it converges on the neutral position. Because of this, it takes time before the steering wheel comes to be converged at the neutral position and stops, during which the vehicle running condition is not stabilized. The effect of this phenomenon is especially large in case of a high speed running.
In order to solve such problem, various proposals have been made such as one described in U.S. Pat. No. 4,754,829 which is designed to effect braking the motor for the steering assistance when the conditions of steering torque and steering rotation speed are satisfied either one or both of which are changed by the vehicle speed, one described in U.S. Pat. No. 4,802,544 which is designed to effect braking by short-circuiting between the two terminals of the motor for assisting steering force when the vehicle speed exceeds a predetermined value and the steering is stopped, and one described in U.S. Pat. No. 4,735,271 which is designed to brake the motor for assisting steering force when the steering torque is no more than a predetermined value, steering rotation speed is more than a predetermined value, and the steering system is at the neutral position.
Also, there has already been proposed one as described in U.S. Pat. No. 4,735,271 which is designed to brake the motor for assisting steering force when the steering torque is no more than a predetermined value and the rotation speed of the motor for assisting steering force is higher than a predetermined value.
However, in those conventional arts, relation between the wheel return control and the wheel convergence control for converging the steering wheel to the neutral position is not considered.
Though the vehicle speed is equal, there are cases, according to the steering conditions, where the wheel return control is necessary and where the wheel convergence control is necessary. The wheel convergence control is necessary, for example, if the wheel rotational speed is high near the neutral position in middle speed running, but the wheel return control is necessary if the wheel rotational speed is low near the neutral position at middle speed running.